Electrolyte for the galvanic deposition of aluminium from aprotic solvents in a plating barrel

ABSTRACT

The invention relates to an electrolyte for the electrodeposition of aluminium from aprotic solvents containing a compound of the formula: N(R 1 ) 4 X.(m-n-o)Al(C 2 H 5 ) 3 .nAlR 2   3 oAlR 3   3 , wherein R 1  is a C 1  to C 4  alkyl group, X is F, Cl or Br, m is equal to 1 to 3, preferably 1.7 to 2.3, n is equal to 0.0 to 1.5, preferably 0.0 to 0.6, o is equal to 0.0 to 1.5, preferably 0.0 to 0.6, R 2 , R 3  is a C 1  or C 3  to C 6  alkyl group, wherein R 2  is unequal to R 3 , in an organic solvent. A further object of the invention is a method for producing the electrolyte, a method for coating and the coated material parts.

The object of the invention is an electrolyte for the electrodeposition of aluminium from aprotic solvents, preferably for the barrel coating of materials. A further object of the invention is to provide a method for producing the electrolyte, the use of said electrolyte, a coating method and parts having been coated using said method.

It is known that the electrodeposition of aluminium is only possible from non-aqueous electrolytes due to the very low electrode potential of this metal.

In principle, electrodeposition of aluminium in non-aqueous systems is carried out by using an electrolyte that contains organic aluminium alkyl complex compounds dissolved in organic aprotic solvents. The coating is carried out in a way that the materials to be coated are operated as a cathode in the electrolyte solution. Furthermore, the electrolyte solution contains aluminium anodes to provide the necessary aluminium for the coating. When applying a current, the aluminium from the aluminium anodes dissolves and is transported through the electrolyte solution towards the materials operated as the cathode and is deposited on them.

Different electrolyte systems have become known for the electrodeposition of aluminium from aprotic solvents. However, in recent years, only electrolyte systems that are based on aluminium alkyl complexes have reached a technological importance for the deposition of aluminium on an industrial scale.

The electrolytic deposition of aluminium from aluminium alkyl complexes was described in the 1950s by Ziegler and Lehmkuhl.

An electrolyte with the composition NaF.2 Al(C₂H₅)₃ is described, for example, in DE 1 047 450 AS. This electrolyte has been used in an aromatic hydrocarbon such as toluene. Deposits of aluminium using this electrolyte are of good quality. However, this electrolyte exhibits poor throwing power. Poor throwing power has the effect that particularly complex shapes with rough corners or edges are not or only insufficiently coated with aluminium. To achieve a uniform coating here, the use of auxiliary anodes is required. However, this method is technically very demanding and costly and cannot be carried out economically on an industrial scale. Therefore, for economically reasonable usage of the electrolyte it should exhibit a sufficient throwing power.

In order to increase the throwing power the aluminium alkyl complexes have been modified. Particularly, sodium fluoride was replaced by potassium fluoride. Due to the exchanged cation, these complexes exhibit a better electrical conductivity and also a better throwing power. However, the considerable drawback of these compounds is that they possess relatively high melting points and, therefore, the solubility of the complexes in the aromatic solvent is low. Thus, such electrolyte solutions tend to crystallize and they become useless due to crystallization of the respective aluminium alkyl compounds, particularly when in storage. Such solutions are difficult to employ particularly on an industrial scale since clogging of pipes, pumps and filters may easily occur due to their tendency to crystallize.

To solve this problem in the prior art the use of potassium fluoride is suggested in mixtures of different aluminium alkyl complexes that have lower melting points than aluminium triethyl complexes. This is supposed to achieve a better solubility of those systems. However, a considerable drawback of these electrolyte systems is that they exhibit a relatively low current density capacity and thus potassium can be easily co-deposited, which is highly undesirable when depositing aluminium. Also, the thermal stability particularly of aluminium triisobutyl complexes is considerably lower as compared to aluminium triethyl complexes. A further drawback is that in industrial operation the mixing ratios of the individual aluminium alkyl compounds have to be adjusted continuously in order to keep them constant.

Moreover, certain aluminium alkyl compounds such as aluminium trimethyl are so expensive that an economic viability of said method is hardly possible.

The electrolytes mentioned are described, for instance, in EP 0 084 816 A1, EP 0 402 760 A1, EP 0 402 761 A1, DE 196 49 000 C2, EP 1 680 533 A1.

These electrolyte systems are suitable for coating complex shapes such as rackplating parts. However, they exhibit serious drawbacks when used in the electrocoating of small parts that are coated in a plating barrel.

Electrocoating of small parts and bulk material is usually carried out in rotating, perforated barrels that are driven by an electric motor and installed in a plastic housing within a supporting frame. The small parts to be coated are introduced into the barrel, followed by dipping the barrel into the electrolyte solution. Trans-mission of current onto the goods to be coated inside the barrel is usually performed by means of copper wires positioned laterally on the barrel. Plating barrels of this type described, for instance, in WO 03/012176 A1 and in WO 2005/021840 A1.

When performing barrel coating of bulk goods the small parts to be coated are coated in rotating, perforated barrels. In doing so, only the portion of the goods is coated which is in direct contact with the perforated wall and which has the shortest distance from the anode. The goods in the interior of the goods package in the barrel are not coated. Thus, the barrel must be rotated very often so that the goods are well mixed and all parts remain sufficiently long enough near the inner perimeter of the barrel in order to achieve a uniform coating.

The surface in the barrel effective for coating is called the enveloping surface. This enveloping surface is very small compared to the total surface of the small parts in the barrel. In comparison to the interior of the barrel, a very high local current density is applied to this enveloping surface, which, depending on the barrel's filling level, is three to tenfold the average current density that is set with respect to the small parts in the barrel.

This means that an electrolyte used for barrel coating has to exhibit a very high current density capacity since the current density distribution in the barrel is not uniform but considerably higher at the enveloping surface than in the interior of the barrel.

For a commercially viable coating of bulk goods in a barrel, an accordingly suitable coating electrolyte has to exhibit a very high current density capacity in order to avoid unnecessarily long operating times when coating in the barrel. If the operating times are too long, the parts may suffer damage due to the barrel's rotation and mutual rubbing, and are furthermore subject to increased abrasion. This is particularly critical with threaded or precision parts.

The organometallic electrolyte systems that have hitherto been employed for aluminium deposition have only a relatively low maximum current density capacity. The drawback here is that undesired side reactions occur and the quality of the coating is heavily affected as soon as a maximum threshold current density is exceeded. Furthermore, the durability of the electrolyte is highly reduced by the high current density. However, frequent changing of the electrolyte causes the coating method to become too expensive and uneconomical.

It has been found that especially electrolyte systems based on complex salts containing alkali metal fluoride, such as potassium fluoride or sodium fluoride, and employed for coating rack-plating parts are not well suited for coating small parts in barrels since they show considerable drawbacks.

Thus, when exceeding the threshold current density due to the high current density capacity co-deposits of alkali metals occur on the small parts to be coated. This hinders deposition of aluminium and the corrosion resistance of the deposited layer is negatively affected. This applies particularly for complexes containing potassium, which have better throwing power than the sodium fluoride complexes.

A further drawback is that sodium fluoride and potassium fluoride aluminium alkyl complexes, when loaded with high current densities, tend to produce a dendritic crystal growth. In this process irregular overgrowths (uncontrolled growth) of aluminium are deposited mostly on the edges of the parts to be coated or on protruding parts of the plating barrel. These dendrites are brittle and ground to loose particles in the barrel and are plated onto the goods to be coated and partially integrated into the coating. This renders the coated products unusable.

For example, deposits in threads of small parts cause impeded joint ability or high friction losses when being screwed together, or the tolerances of the small parts are not adhered to.

A further drawback of the hitherto existing electrolyte systems based on potassium fluoride or sodium fluoride is that these complexes can only be employed at very low average current densities due to their low current density capacity. Thereby, long retention times inside the plating barrel are necessary in order to achieve the desired layer thicknesses. Hence, abrasion on the parts and also on the deposited layer is very high and the quality of the layer obtained too low.

The complexes containing sodium fluoride, which are basically more suitable than potassium fluoride complexes due to their current density capacity, also have poor throwing power for the coating of bulk goods or small parts, such as screws or hollow rivets. Thus, said complexes are not suitable for electrocoating in barrels either.

Furthermore, so called onium complexes are known as electrolytes from older documents of the prior art dating from the years 1959-1967. Onium complexes are complexes in which sodium or potassium fluoride is replaced by tetraalkylammonium halogenides. These electrolyte complexes were described in the 1960s, but never used on an industrial scale because the deposits caused a non-uniform pattern.

DE 10 56 377 AS, for example, describes the employment of an electrolyte composition N(C₂H₅)₄Cl.2Al(C₂H₅)₃ for depositing aluminium. However, these electrolyte complexes were employed directly and without organic solvents.

Said complexes have the disadvantage that in this form they are not suited for industrial application due to the high viscosity of the liquid compounds. Due to the high viscosity this can lead to a carry-over of the electrolyte and, therefore, because of the high inflammability of the electrolyte, to an increased safety risk. Moreover, the electrolyte in this form is also not suitable for electrolysis in a plating barrel because a relatively poor exchange of the electrolyte between anode and cathode occurs due to its high viscosity and there is a risk that the deposition will completely cease due to a depletion of the electrolyte in the barrel.

Similar electrolytes have also been described in DE 14 96 993 AS. These exhibit alkyl ammonium compounds with benzyl, phenyl or cyclohexyl groups as well as highly branched hydrocarbon moieties. These electrolyte compounds are also employed in undiluted form as solutions so that the same drawbacks apply as already mentioned above. Furthermore, the electrical conductivity of these electrolyte systems is very low so that they are not suitable for electrolysis in plating barrels. For example, the specific electrical conductivity of the electrolyte [N(C₂H₅)₃(C₆H₅CH₂)]Cl.2Al(C₂H₅)₃ is 8.6 mS/cm. This conductivity is not sufficient for coating in a plating barrel.

Therefore, the technical goal of the invention was to provide an electrolyte systern ideally suitable for depositing aluminium on small parts in a plating barrel.

This technical goal is achieved by an electrolyte for electrodeposition of aluminium from aprotic solvents containing a compound of the formula

N(R¹)₄X.[(m-n-o)Al(C₂H₅)₃ nAlR² ₃ .oAlR³ ₃],

wherein R¹ is a C₁ to C₄ alkyl group,

X is F, Cl or Br,

m is equal to 1 to 3, preferably 1.7 to 2.3, n is equal to 0.0 to 1.5, preferably 0.0 to 0.6, o is equal to 0.0 to 1.5, preferably 0.0 to 0.6, R², R³ is a C₁ or C₃ to C₆ alkyl group, wherein R² is unequal to R³, in an organic solvent.

As opposed to the hitherto use of electrolytes containing sodium fluoride and potassium fluoride, the electrolytes of the invention have the advantage that they do not show any alkali metal co-deposition, especially at high current densities, and this thereby guarantees a uniform aluminium coating without losses in corrosion resistance.

Furthermore, these electrolytes do not show any tendency to produce dendritic growth. This too leads to an increase in quality of the aluminium layer on the product to be coated.

In addition, these electrolytes have a high current density capacity and good conductivity. As a result of the high current density capacity in plating barrels, a relatively high average current density becomes possible with regard to the total surface of the goods to be coated. Furthermore, it is an advantage that the time required for coating in the barrel is considerably lower and, therefore, there is a lower risk that damages to and abrasions on the parts to be coated occur due to the barrel's rotation.

The electrolytes according to the invention also possess a very high throwing power, which in addition allows more complex shapes to be well coated.

Furthermore, it was surprising that by slightly diluting the electrolyte with an organic solvent, preferably toluene, an increase in the system's conductivity could be achieved by increasing the ion mobility from two to four mS/cm measured at 95° C. This was a complete surprise for the expert since he had to reckon with the fact that a dilution of the electrolyte would lead to a decrease in conductivity.

In a preferred embodiment, the electrical conductivity of the electrolyte of the invention is greater than 25 mS/cm at 95° C., preferably between 28 and 35 mS/cm at 95° C.

The conductivity is measured using a commercially available conductivity probe (e.g. TetraCon 325 Pt by WTW Inc.) in a glass vessel with 25 ml of electrolyte in an atmosphere of argon, kept at 95° C. in an oil bath.

Preferably, a solvent is used as an organic solvent which is selected from the group containing toluene, xylene or benzene, or mixtures thereof. In the electrolyte the solvent is present in a concentration of 1 to 4 mol, preferably 2 mol, per mol of complex compound. It is particularly preferred when the compound of the General Formula I is as follows: N(C₂H₅)₄Cl.2Al(C₂H₅)₃2 toluene or N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5 Al(CH₃)₃.2 toluene or N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5 Al(C₄H₉)₃ 2 toluene.

Another object of the invention is a method to produce the electrolyte. First, the tetraalkylammonium halogen compound is dried in order to remove humidity. Then, the dried substance is suspended in an organic aprotic solvent, preferably toluene. Subsequently, the aluminium trialkyl compounds or mixtures of aluminium trialkyl compounds are added dropwise during cooling until a clear solution of the end product is obtained. The electrolyte is preferably used for the electrodeposition of aluminium on material parts. These material parts are selected from metals, alloys, ceramics, plastics, or composite materials made of one or more of said materials. Preferably, the material parts to be coated are placed into a coating barrel and coated with aluminium in said coating barrel.

The coating process is carried out in several steps. First, the parts to be coated, which have been pre-treated if necessary, are placed in the coating barrel. The coating barrel is dipped into the electrolyte of the present invention. Then, a cathodic current is applied to the coating barrel, and an anodic current is applied to the aluminium anodes, which have been placed into the electrolyte solution. This results in an aluminium layer being deposited onto the parts to be coated, and the aluminium anodes being dissolved. Afterwards, the parts are removed from the coating barrel and subsequently dried.

Examples for typical operating parameters under which the electrolytes of the invention are employed in plating barrels are: an operating temperature of the electrolyte solution of 90 to 100° C., a cell voltage of 10 to 40 V, and an average current density of 0.4 to 1.0 A/dm², wherein the current density is 4 to 6 A/dm² at the enveloping surface of the plating barrel. The speed of deposition on the materials to be coated is 10 to 12 μm per hour, with an average current density of 1 A/dm².

A further object of the invention are parts coated with aluminium, produced using the method according to the invention.

The electrolyte of the invention exhibits considerable advantages regarding dendritic growth particularly in comparison to the electrolytes of the prior art, which contain sodium fluoride or potassium fluoride. A series of experiments with an electrolyte having the composition NaF.2Al(C₂H₅)₃.2 toluene was carried out. When coating using said electrolyte a strong dendritic edge growth occurred on the gap plates that were used as test specimens. When using an electrolyte of the invention, e.g. N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene, it was found that no dendritic growth occurred and a smooth surface layer was achieved on the edge. This too proves the superiority of the electrolyte of the invention as opposed to the hitherto used electrolytes containing sodium fluoride or potassium fluoride.

The maximum current density capacity of the electrolyte according to the invention is in the range of 5 to 6 A/dm², whereas the current density capacity of the respective electrolyte containing potassium fluoride is in the range of 1 to 1.5 A/dm², the electrolyte containing sodium fluoride is in the range of 3 to 4 A/dm² and the electrolyte containing ammonium benzyl is at 1.5 A/dm². Due to the high current density capacity of the electrolyte of the invention a commercially viable deposition is possible in a plating barrel in a short time, without the parts being damaged. Thus, a high quality coating is achieved.

The following examples are to explain the invention in more detail.

EXAMPLES 1. Preparation of the Electrolyte

Tetraethylammonium chloride, which can be purchased as monohydrate, is dried in a vacuum. 1 mol of the dried tetraethylammonium chloride is mixed with toluene so that a suspension is obtained. 2 mol of triethylaluminium, which is used either as a pure substance or dissolved in toluene, is added to this suspension dropwise during cooling. The reaction mixture is stirred. A clear solution of N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene is obtained. 2 mol of toluene are used as a solvent with respect to the employed amount of tetraethylammonium chloride. The obtained product is used as an electrolyte.

2. Conductivity of the Electrolyte According to the Invention as Opposed to the Electrical Conductivity of Electrolytes of the Prior Art

A sufficient electrical conductivity of the electrolyte is a basic requirement for a sufficient coating, particularly of small parts in plating barrels. The conductivity of the electrolyte of the invention is substantially higher than those of the prior art, which contain sodium fluoride or potassium fluoride. Surprisingly, it was found that a toluene-free electrolyte has a lower electrical conductivity than the electrolyte of the invention, which contains 2 mol of toluene. The following table shows a comparison of the individual conductivities of the electrolytes of the invention and of the electrolytes of the prior art.

TABLE 1 Conductivity Electrical conductivity Compound [mS/cm] 95° C. N(C₂H₅)₄Cl•1.5 Al(C₂H₅)₃•0.5 Al(CH₃)₃•2 toluene 31-32 N(C₂H₅)₄Cl•2 Al(C₂H₅)₃•2 toluene 29-31 NaF•2 Al(C₂H₅)₃•2 toluene 26-27 KF•2 Al(C₂H₅)₃•2 toluene 24-26 N(C₆H₅CH₂)(C₂H₅)₃Cl•2 Al(C₂H₅)₃ 8.6

3. Coating of Gap Plates

In the method of the invention gap plates made of brass were used in coating experiments. These gap plates had the following dimensions: width 20 mm, stretched length 100 mm. Prior to coating, 25 mm of the lower region of the gap plate was bent 180°, resulting in a plate with a J-shaped longitudinal section, with a gap width of 1 mm between the two branches. After coating, the extent of the aluminium deposition in the gap between the two branches after bending the plate, and thus the throwing power of the electrolyte system, could be assessed and compared to further coating experiments with different coating parameters or electrolyte compositions.

Anodes with a width of 15 mm were used as aluminium anodes to particularly allow experiments concerning long time deposition. As electrolyte N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene was used. Furthermore, in the experiments the cathodic and anodic yields were calculated as a function of the load of the electrolyte with current (in Ah/L). The electrode effects visible at the anodes, such as gas development and plate-out, were evaluated.

Coating Experiment 1

A gap plate was coated for 30 min at a current of 350 mA (corresponding to a current density of 2.5 A/dm²) and a voltage of 2.4 V. Immediately after switching on the coating current the coating uniformly attached to the plate. The coating was white, smooth, uniform and matt. No dendrite forming occurred. Even under the microscope the edges were smooth. The inner surface was also completely coated. The electrolyte exhibited an excellent throwing power. The structure of the coating was microcrystalline and of high quality. No gas development was observed at the aluminium anodes.

Coating Experiment 2

Another coating of a gap plate was carried out within 30 min at a current of 450 mA (corresponding to a current density of 3.2 A/dm²), at a voltage of 2.4 V. The same electrolyte as in Example 1 was used. Again, white smooth satin-gloss layers were produced that did not exhibit any dendrite forming at the edges. The inner surface of the gap plate was also completely coated. The structure was microcrystalline and no gas development was observed at the anodes.

Coating Experiment 3

In a further coating experiment in an electrolyte composed of N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5Al(CH₃)₃.2 toluene a gap plate was again coated for 30 minutes at a current of 450 mA (corresponding to a current density of 3.2 A/dm²). A smooth, silvery layer with slight shadings was produced, without rough spots or dendritic growth at the edges and points.

Coating Experiment 4

In the electrolyte of the composition of Experiment 3, a gap plate was coated for 30 minutes at a current of 600 mA (corresponding to a current density of 4.3 A/dm²) and a voltage of 2.78 V. The layer thickness achieved was about 25 μm. The layer was silvery with slightly glittering areas and microcrystalline without dendrites at the edges and points. The plate was completely coated at the edges.

Further Coating Experiments

Further coating experiments were performed with the following electrolytes of the invention: N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene, N(C₂H₅)₄Cl.1.5 Al(C₂H₅)₃.0.5Al(CH₃)₃.2 toluene and N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5Al(CH₃)₃.2 toluene. In all cases, high-quality layers were produced. Also, the inner surfaces of the gap plates were sufficiently coated. No dendrite forming occurred at the edges of the gap plates and no gas developed at the anodes.

Examination of Dendrite Growth

Comparative experiments concerning dendritic growth were performed with the electrolyte of the invention, i.e. N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene, and an electrolyte of the prior art, i.e. NaF.2Al(C₂H₅)₃.2 toluene. The coating was carried out using the same current, voltage and time conditions for both electrolytes. The coating was performed at 350 mA for a coating time of 60 min, particularly in order to allow an easier comparison of edge growth. The microscopic exposures of the edges, which are depicted below, reveal significant differences when comparing the electrolyte of the prior art, containing sodium fluoride, to the electrolyte of the invention. The coating created by the electrolyte of the prior art exhibited a clearly visible dendritic growth at the edges and no smooth structure (see FIG. 1). The electrolyte of the invention did not exhibit any dendritic growth at the edges but had a clear smooth coating (see FIG. 2).

These dendrites are extremely harmful for a coating. They are brittle and are ground to loose particles, particularly during electrocoating in barrels, and cement themselves during the coating process forming deposits on the goods to be coated. This deposit-forming and the partial integration into the aluminium layers renders the products unusable because, for example, deposits in screw threads of the goods cause an impeded joint ability of the thread or very high friction losses. Moreover, dimensional tolerances can no longer be adhered to.

The electrolyte according to the invention does not show any dendritic growth and can produce an aluminium layer which is smooth and of high quality and which does not exhibit the drawbacks described above.

Long-Term Stability of the Electrolyte

The long-term stability of the electrolyte of the invention, N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene, was also examined. A total of three anode sets were consumed. Coating times of up to 64 hours at a current density of 0.63 to 2.2 A/dm² were applied. The total load of the electrolyte after the first set of anodes was 417 Ah/L with eight coatings and 748 Ah/L with five further coatings after the second set of anodes.

In these long-term experiments it became visible that the anodic yield of the system was approximately 100% and that regardless of the long shelf life of the electrolyte sufficient and high quality-coatings could be achieved in all experiments. This shows that the electrolyte of the invention, N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene, is suitable even for industrial applications due to its long-term stability and because it can be used for many coating cycles.

Furthermore, it was found that during a longer idleness of the plant and after cooling of the electrolyte down to 15° C. no crystallization in the electrolyte had occurred, thus the stability of the electrolyte could be verified here as well.

The electrolyte of the invention therefore is a reasonable improvement on hitherto employed electrolytes based on sodium fluoride or potassium fluoride because it exhibits better properties and leads to aluminium deposits of higher quality. Thus, a co-deposition of sodium or potassium is not possible with the electrolyte of the invention. No dendrite forming can be observed. The current density capacity and the throwing power of the electrolyte is very high, and the electrolyte of the invention furthermore also has a high long-term stability so that it can be used cost-effectively in large-scale industrial processes. 

1. An electrolyte for the electrodeposition of aluminium from aprotic solvents containing a compound of the formula: N(R¹)₄X.(m-n-o)Al(C₂H₅)₃ .nAlR² ₃ .oAlR³ ₃,  (I) wherein R¹ is a C₁ to C₄ alkyl group, X is equal to F, Cl or Br, m is equal to 1.7 to 3, preferably 1.7 to 2.3, n is equal to 0.0 to 0.6, o is equal to 0.0 to 1.5, preferably 0.0 to 0.6, R², R³ is a C₁ or C₃ to C₆ alkyl group, wherein R² is unequal to R³ in an organic solvent.
 2. The electrolyte according to claim 1, wherein the electric conductivity of the electrolyte is greater than 25 mS/cm at 95° C.
 3. The electrolyte according to claim 1 wherein the organic solvent is selected from the group consisting of toluene, xylene or benzene and mixtures thereof.
 4. The electrolyte according to claim 1, wherein the solvent is at a concentration of 1 to 4 mol per mol of compound.
 5. The electrolyte as claimed in claim 1, wherein the compound has the composition N(C₂H₅)₄Cl.2Al(C₂H₅)₃.2 toluene.
 6. The electrolyte as claimed in claim 1, wherein the compound has the composition N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5Al(CH₃)₃.2 toluene or N(C₂H₅)₄Cl.1.5Al(C₂H₅)₃.0.5Al(C₄H₉)₃.2 toluene.
 7. A method to produce the electrolyte as claimed in claim 1, comprising the following process steps: i) drying of the compound N(R¹)₄X in order to remove humidity, ii) preparing a suspension from the dried compound N(R¹)₄X in an organic aprotic solvent, and iii) adding the aluminium alkyls or the mixtures of aluminium alkyls by dropwise addition during cooling until a clear solution is obtained, wherein R¹ is a C₁ to C₄ alkyl group, and X is equal to F, Cl or Br.
 8. A method for the electrodeposition of aluminium on material parts comprising placing the material parts in the electrolyte according to claim 1, wherein aluminum is electrodeposited on said material parts.
 9. The method claimed in claim 8, wherein the material parts are selected from the group of materials consisting of metals, alloys, ceramics, plastics, and composite materials made of one or more of said materials.
 10. The method as claimed in claim 8, wherein the material parts to be coated are placed inside a coating barrel and are coated with aluminium therein.
 11. A method for coating material parts with aluminium, comprising the following process steps: i) placing the parts to be coated into a coating barrel, ii) dipping the coating barrel into an electrolyte according to claim 1, iii) applying a cathodic current to the coating barrel and an anodic current at the Al-electrodes that are located according to claim 1, wherein aluminium de-posits itself on the parts present in the coating barrel, iv) coating the parts with aluminium, v) removing the parts from the coating barrel and drying the parts.
 12. Aluminium-coated parts produced by the method according to claim
 11. 